Expression of allene oxide synthase determines defense gene activation in tomato

Abstract

Allene oxide synthase (AOS; hydroperoxide dehydratase; EC 4.2.1.92) catalyzes the first step in the biosynthesis of jasmonic acid from lipoxygenase-derived hydroperoxides of free fatty acids. Using the AOS cDNA from tomato (Lycopersicon esculentum), in which the role of jasmonic acid in wound-induced defense gene activation has been best described, we examined the kinetics of AOS induction in response to wounding and elicitors, in parallel with that of the wound-inducible PIN II (proteinase inhibitor II) gene. AOS was induced in leaves by wounding, systemin, 12-oxophytodienoic acid, and methyl jasmonate. The levels of AOS mRNA started declining by 4 h after induction, whereas the levels of PIN II mRNA continued to increase up to 20 h after induction. Salicylic acid inhibited AOS and PIN II expression, and the addition of 12-oxophytodienoic acid or methyl jasmonate did not prevent the inhibition of PIN II expression in the presence of salicylic acid. Ethylene induced the expression of AOS, but the presence of ethylene alone did not produce an optimal induction of PIN II. The addition of silver thiosulfate, an ethylene action inhibitor, prevented the wound-induced expression of both AOS and PIN II. Products of hydroperoxide lyase affected neither AOS nor PIN II, but induced expression of prosystemin. Based on these results, we propose an updated model for defense gene activation in tomato.

Figure 1

a, Nucleotide and deduced amino acid sequence of the tomato AOS cDNA. The large arrow indicates the transcription start site and the small arrow indicates the point at which homology with other published AOS sequences start. The asterisk indicates the proposed cleavage site of the mature leader sequence. The amino acids constituting the four Cyt P450 domains at the C terminus of the sequence are underlined, and the primers used for amplifying the coding region for the fusion protein are double-underlined. b, Alignment of amino acids constituting the Cyt P450 domains A through D in flax, rubber, Arabidopsis, and tomato AOS proteins.

Figure 1

a, Nucleotide and deduced amino acid sequence of the tomato AOS cDNA. The large arrow indicates the transcription start site and the small arrow indicates the point at which homology with other published AOS sequences start. The asterisk indicates the proposed cleavage site of the mature leader sequence. The amino acids constituting the four Cyt P450 domains at the C terminus of the sequence are underlined, and the primers used for amplifying the coding region for the fusion protein are double-underlined. b, Alignment of amino acids constituting the Cyt P450 domains A through D in flax, rubber, Arabidopsis, and tomato AOS proteins.

Figure 2

a, Southern blot of tomato genomic DNA restricted with NotI, EcoRI, and HindIII and probed with the tomato AOS cDNA. b, Expression pattern of AOS in different plant parts of tomato.

Figure 3

Induction of tomato AOS by wounding, systemin (Sys), PDA, and MeJA. Tomato seedlings (16–21-d-old) were wounded, exposed to MeJA, or the cut stems were placed in PDA or systemin solutions. In the case of PDA and systemin, the shoots were transferred to water after the initial 2 h of immersion in the treatment solutions. Seedling shoots were harvested 0, 1, 4, or 20 h after application of treatments, and lanes 1 to 4 represent these time points, respectively.

Figure 4

Effect of products of HPL on PIN II (a and b) or prosystemin (c) gene expression. Tomato seedlings were exposed to trans-2 hexenal (t-2-h), cis-3-hexenal (c-3-h), traumatin, traumatic acid (TA), or wounded as described in “Materials and Methods.” t-2-h, c-3-h, and MeJA were applied to cotton swabs placed within sealed glass jars containing the potted plants. Traumatin and TA were sprayed on the plant as solutions prepared in potassium phosphate buffer (15 mm, pH 6.5) containing 0.01% (w/v) Triton X-100. Seedlings were harvested 4 h after starting exposure to treatments. Methanol was used as a solvent for MeJA and t-2-h was used as a control, as was the zero-time point.

Figure 5

Effect of SA on the expression of AOS, PIN II, and PR1b1 in tomato. Lane 1, Buffer control; lane 2, SA; lane 3, wounding; lane 4, wounding plus SA; lane 5, PDA; lane 6, PDA plus SA; lane 7, MeJA; lane 8, MeJA plus SA; lane 9, systemin; and lane 10, systemin plus SA. The concentrations used for the various treatments and the method of treatment application are given under “Materials and Methods.” Seedlings were pretreated with SA for a period of 2 h prior to wounding or application of elicitors. Harvesting was done 4 h after treatment application.

Figure 6

Effect of ethylene on the expression of AOS and PIN II in tomato. Lane 1, Buffer control; lane 2, wounding; lane 3, ethephon; lane 4, AVG; lane 5, AVG plus wounding; lane 6, STS; lane 7, STS plus wounding; lane 8, STS plus ethephon; lane 9, SA plus ethephon. Concentrations of the chemicals and the method of application are described in “Materials and Methods.” Pretreatment with AVG, STS, or SA was for 2 h, after which time the treatments were applied. Seedlings were harvested 4 h after treatment application.

Figure 7

An updated model proposed for defense gene activation in tomato. Wounding, systemin, and other signals act through ABA on a phospholipase to release free fatty acids, which form the substrate for lipoxygenase. The fatty acid hydroperoxides produced by phospholipase are used by AOS to form PDA and JA, or by HPL to form six-carbon volatiles and traumatin. PDA and JA activate defense gene expression. The C6 volatiles and traumatin induce expression of prosystemin. Processing of prosystemin to systemin presumably requires an unidentified factor that also responds to wounding. The processed systemin then acts as a systemic intercellular signal. Ethylene, produced in response to wounding, induces the expression of AOS. Ethylene together with JA is required for the optimum expression of PIN II. STS blocks the ethylene- and wound-induced expression of AOS and PIN II. SA inhibits the effect of ethylene on AOS induction, inhibits the expression of AOS, and inhibits the action of PDA and JA in defense gene induction.